Straight stitches in a lock stitching are divided into normal stitches and hitch stitches. If those are mixed, the stitches are disoriented. In general, it is therefore desirable to carry out stitchings with the normal stitches only which do not cause untwistings of a stitched thread.
The straight stitches and the hitch stitches are formed in dependence upon whether a needle drops at a left side or a right side of the stitching direction with respect to the lower thread drawn from a loop taker.
When a hook of the loop taker 28 is positioned before the lower thread drawing point seen from the stitching forward direction, that is, when the lower thread is positioned at the left side of the needle, the straight stitches are formed. Reversely, when the hook is positioned after the lower thread drawing point, that is, when the lower thread is positioned at the right side of the needle, the hitch stitches are formed.
Therefore, in order to provide for forming straight stitches the lower thread is supplied at the left side, and the hitch stitches are prevented thereby.
However, depending upon this practice in the zigzag stitching, the lower thread is biased, especially when the stitching pitches are made small. In order to prevent such cases and form stitches without biasing, it is necessary in the prior art that the lower thread drawing point is, determined at the center in the amplitude of the zigzag stitching.
As shown in FIG. 16, if the lower thread 40 is positioned on the left side of the needle from a view of a machine operator, the loop taker is rotated counter-clockwise as shown by an arrow, and catches a needle thread loop formed on the right side of the lower thread and locks the needle (upper) thread to the lower thread through a stitch formation phases shown in FIG. 16 (a)-(g). As the result, the straight stitch is formed. When the lower thread is positioned on the right side of the needle as shown in FIG. 17, the loop taker is rotated to catch a needle thread loop formed on the left side of the lower thread and locks the needle thread to the lower thread through a stitch formation phases shown in FIG. 17(a)-(g). As the result, the hitch stitch is formed.
As particularly shown in FIG. 19a, the lower thread is supplied to the left end L of the laterally elongated needle dropping hole from a position opposite to the needle hole and leftward of the left end L of the needle hole, because the straight stitches are generally formed with a needle position set adjacent to the left end L of the laterally elongated needle hole 18a within which the needle is laterally swingable from minimum to maximum for zigzag stitches. The needle position adjacent to the left end L of the needle hole is so set as to reduce the up and down movements of a fabric which may otherwise be caused as the needle penetrates into and out of the fabric to be sewn.
However with reference to FIG. 15(a), in case of zigzag stitching, the needle is swingable between the opposite needle positions L and R over a distance D. It is therefore observed that the required amount of lower thread is different depending upon the needle positions L and R. Much more amount of lower thread is required when the needle come to the right end needle position R. The thread tension due to drawing out an additional amount of lower thread will pull the needle thread down onto the underside of the fabric to be sewn.
To prevent the lower thread from biasing and hitch stitching, the lower thread drawing opening is made different in the straight stitching and the zigzag stitching.
Laid open Japanese utility model No. 23,074/81 assigned to the assignee of the present application discloses that, with respect to a sewing machine having the lower thread drawing opening at an amplitude center, a member moving the lower thread drawing open to the left side of the needle for carrying out the straight stitching, is connected to a pattern selecting member for guiding the lower thread to the left side.
In conventional devices the guide member is moved laterally for selecting the patterns so as to make the guide member act directly on the lower thread and guide the lower thread drawing open. This is excellent in clearly switching the lower thread drawing open. When a guide of a lower thread is released, the lower thread is bent and is slackened by a guided amount, and if this amount is absorbed when consecutive stitches are formed, the slackening is cancelled from the thread. But due to the slackening of the thread, the stitches just after having changed the pattern is sometimes to disoriented.